More than 28 percent of U.S. homeowners owed more than their properties were worth in the first quarter, as housing values fell the most since 2008, Zillow Inc. said today. Homeowners with negative equity increased from 22 percent a year earlier, as home prices slumped 8.2 percent during the past 12 months, Bloomberg reported after the release of Zillow’s housing data. About 27 percent of homes were “underwater” in the fourth quarter, according to Zillow, which runs a website with property value estimates and real estate listings. Home prices fell 3 percent in the first quarter and will drop as much as 9 percent this year as foreclosures spread and unemployment remains high, said Stan Humphries, Zillow’s  chief economist. Prices won’t find a floor until 2012, he said.